NullReferenceException: Object reference not set to an instance of an object test_Movement.Update () (at Assets/Scripts/Ai/test_Movement.cs:39)

First of all hello to everybody,

i am new to unity and programming so i tryed to make one early script of a player movement, the player should follow the mouseclick.

This is how my script looks like:

using UnityEngine; using System.Collections;

public class test_Movement : MonoBehaviour { // variables NavMeshAgent navAgent;

 // functions
 void start()
 {
     navAgent = GetComponent<NavMeshAgent>();
 }
  
 
 
 
 

 //Functions
     // Update is called once per frame
     void Update ()
 {
     RaycastHit Markierung;
     Ray MarkierungRay = Camera.main.ScreenPointToRay(Input.mousePosition);



     if (Input.GetMouseButtonUp(0))
     {
         if (Physics.Raycast(MarkierungRay, out Markierung, 100))
         {
             //Debug.Log(Markierung.point);
             navAgent.SetDestination(Markierung.point);
         }
     } 
 }

}

now the problem is if i try to run it, an error appears:

NullReferenceException: Object reference not set to an instance of an object test_Movement.Update () (at Assets/Scripts/Ai/test_Movement.cs:39)

so somthing is missing, but i have no clue what it could be? if i use the debug.Log it works perfectly i allready search for simmilar problems here did not get the answere that helps me

thanks to every help greeting CD